Aquatic adaptations occur in those animals which live in water habitat, viz., fresh, brackish or sea water. They are called aquatic animals or hydrocoles. Based upon the phylogenetic history of the aquatic animals, following two types of hydrocoles have been recognised :

1. Primary aquatic animals :-

  • The primarily aquatic animals are those in which the phylogenetic history is restricted to water as habitat. Therefore, all their adaptations are originally de signed to meet the necessities of aquatic life. Generally, by Adaptations Pit Viper has heat sensitive pits between each eye and nostril. This enables them to pick up changes in temperature around them. (Aquatic Adaptations, Volant Adaptations And Desert Adaptations) primarily aquatic forms is meant the fishes, which have never had a terrestrial ancestry, but have evolved from more primitive aquatic progenitors. As a consequence their adaptation to a dense watery medium is perfect and they do not suffer as those secondarily adapted do through their inability to breathe water. They are, therefore, the primitive gill breathing vertebrates.
  • Primary aquatic animals include protozoans, sponges, coelenterates, some annelids, molluscs and arthropods ; echinoderms ; and chordates such as cephalochordates (Amphioxus), urochordates (Herdmania, Doliolum, Salpa, etc.), cyclostomates (Petromyzon, Myxine), fishes, etc.

Secondary aquatic animals :-

  • These are those hydrocoles which have a record of terrestrial life in their phylogeny (i.e., they are descended from ancestors which led a life on land). Secondary aquatic animals are lung-breathers, mainly amphibious vertebrates, which through stress of circumstances such as inhospitable lands, where food was scarce or severe competitions were forced to return once more to the water.
  • Consequently these animals show in their body structure, the evidences which speak of their ancestry from land-living animals, e.g., Pila, frog and other amphibians, swamp river turtles (Notosaurs, Phytosaurs), crocodiles, birds such as Ichthyornis, penguins, albatrosses, petrels, ducks, geese, etc., and mammals such as Hippopotamus, Otter, whales, porpoises, etc.

Primary Aquatic Adaptations

Body contour :-

  • The form of body depends upon the habits of life. The majority of fixed and partly sedentary forms have radially symmetrical body forms, e.g., sponges (Sycon, Euplectella, Hyalonema, etc.,), Hydra, Obelia, Aurelia, Metridium (sea anemone), echinoderms such as Holothuria, Echinus, Astropecten or star fish. The active locomotor type have fusiform, spindle-shaped elongated and worm like bodies. The spindle form is the characteristic of fishes and wavy, worm-like form is found in the annelids (Nereis).
  • The piscine body is designed for fast loco[1]motion in water. There occurs a side to side com[1]pression of head, body and tail into a beautifully curved streamlined fish form. Head is sub-conical. There is no protuberance over the body, which would retard the swift passage of the animal through water. Further, since the relative weight is more on the upper half of the fish body (on account of the myotomes) a dead fish will float ventral side turned upwards.
  • Regarding the body form of planktonic or ganisms (which float passively at or about the surface), some are globular such as Noctiluca, some have umbrella shape (medusa of Obelia, Aurelia) and some have barrel-like shape (tunicates such as Doliolum and Salpa).

Swimming organs

  • In vertebrates, the primary aquatic animals are the fishes. The fish move (swim) by the help of fins and also by lateral undulations of the flexible body. The fins of fish are of two types, the median fins and paired fins.
  • The median fins include dorsal fins, caudal fins and ventral fins. The paired fins are the pectorals and pelvics. The caudal fin is the chief propeller, the dorsal and ventral fins help to keep body vertical and the pectoral and pelvic fins help in propulsion and in making changes in direction.

Respiration :- The primary aquatic animals are able to respire inside the water, without the need to come up to the surface. The exchange of respiratory gases takes place between the blood of these animals and the water outside. There are two methods of aquatic breathing : 1. through diffusion through general body surface, e.g., protozoans, coelenterates and planktonic larvae, and 2. with the help of special organs called branchia or gills, e.g., prawns, and other crustaceans; Unio, Pila and other molluscs ; and many vertebrates such as fishes, tadpole of frog and salamanders. Indeed, gills of fishes are most remarkable aquatic breathing organs utilizing dissolved oxygen of water.

Air bladder :- Advance bony fishes contain air bladder (or swim bladder) which serve as an accessory respiratory organ and hydrostatic organ. Air bladder is a hollow outgrowth of the alimentary canal and is filled with gas or air.

Lateral line sense organs :- Fishes have lateral line systems extending all over the body. It contains neuromast organs which act as rheoreceptors (i.e., detect pressure changes in surrounding water).

Skin :- Skin of fishes is rich in mucous glands and/or is protected with scales.

Secondary Aquatic Adaptations

Stream-lined body :- The body shape is stream-lined like primarily adapted forms : neck constriction disappears and tail enlarges, e.g., Ichthyosauria (extinct fish-lizards), Cetacea (whales, dolphins, and porpoises), Sirenia (manatees and dugongs), Pinnipedia (walrushes and seals). Frog also contains stream-lined body. 

Enlargement of size :- Aquatic vertebrates tend to be larger in size because in these creatures energy, which in terrestrial form is exhausted in gravitational forces, is turned into growth. For example, largest sulphur-bottom whale (Balaenoptera musculus) is several times bigger than the largest elephant. Other example include giant sharks and squids. 

Submergence :- All secondary aquatic animals need to develop capacity of submergence since swimming below water surface demands such an adaptation. For example, in whales the ribs are strongly arched, the lungs are massive, the external nostrils communicate with the median “blow hole” which is closable. Certain adult aquatic insects too are able to increase their period of submergence by storing air inside the subelytral space, e.g., Nepa. 

Shortening of neck :- There occurs reduction of length and mobility of neck. In whales cervical vertebrae (which are seven in number like other mammals) are fused to form a solid and compressed mass of bone.

Disappearance of excrescences :- The external ears (pinnae) which hinder water locomotion tend to disappear, since they collect sound waves in air medium and are useless in aquatic forms. Thus, ears are reduced in amphibious mammals and are lost in whales, true seals and walruses. The nostrils (nares) move towards the apex of head as in whales, ichthyosaurs, phytosaurs, etc. Nares are often capable of being closed (e.g., otter). Likewise, eyes become water-adapted by shifting higher on the face as in hippopotamus.

Occurrence of locomotory paddles (fins) :- There occur fleshy and fin like expansions of the body wall in whales and ichthyosaurs which help in propulsion. These fins may be dorsal or caudal. Dorsal fin is present in killer whale, while absent in Delphinopterus and Balaena. Caudal fin (also called caudal or tail fluke) of marine mammals in horizontal (vertical in reptiles) and the bone divides the tail into two equal parts rather than running into one lobe. In turtles oar propulsion occurs by fin-like limbs; but in whales, sirenians, etc., tail propulsion takes place as their hindlimbs become disappeared. Pectoral paddles of whales and sirenians exhibit the following adaptations :

  • The restriction of movements corresponding to the elbow and wrist joints.
  • The fusion between digits.
  • Increase in the number of phalanges, called hyperphalangy
  • Increase in the number of digits for increase of expanse of paddling surface, called hyperdactyly.

Disappearance of hairs, skin glands, etc :- In whales and sirenians, the skin becomes naked due to loss of hairs. The hair loss is compensated by the formation of a fatty layer below the skin (blubber) for the retention of the bodily heat. The blubber also has a hydrostatic advantage (e.g., it helps in floatation or to keep positions in the water and act in combination with the buoyancy of the aquatic medium). Sweat or oil glands disappear as they have nothing to do with the aquatic mode of life. Muscles and nerves also atropy from the integument due to its thickening and immobility.

Mouth arangement :- Since jaws are not used for mastication in whales they lost the power of movement. Teeth become simplified (homodont in dolphins) and greater in number. In sperm whale, teeth are present only on one jaw or entirely absent from both the jaws (e.g., baleen whale).

Skull modification :- In certain aquatic mammals (e.g., Dolphins, porpoises) the cranium is shortened and front part of the skull becomes elongated to acquire the shape of a rostrum. In the skull of the cetacea, the zygomatic arch is reduced to a vestige.

Simplification of vertebrae :- In secondarily aquatic forms (vertebrates) the vertebrae tend to be simple. In Ichthyosaurs, vertebrae are simple with biconcave centra like the fishes. Various secondary articulations or zygapophysis become reduced, as body weight is supported by water. The chest too become cylindrical. The rib articulations are modified and are central, i.e., they are articulated to the centrum and are not articulated to the transverse processes. Sacrum in cetaceans and sirenians is more or less reduced, since it does not withstand and transmit the supporting impact of the hindlimbs, as does in terrestrial forms.

Lightness of bones :- The bones in aquatic forms are light and spongy. In whales, their interstices are filled with oil.

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